1. Field of the Invention
The present invention relates to a thread take-up lever driving device in a sewing machine such as an embroidery machine.
2. Description of the Prior Art
In a conventional thread take-up lever driving device, a normal sewing operation is performed in such a manner that a thread take-up lever is pivoted downwardly to loosen an upper thread when the upper thread is engaged by a hook of a shuttle and is withdrawn into the shuttle; the thread take-up lever is pivoted upwardly to tighten the upper thread after the upper thread has been removed from the shuttle; and the thread take-up lever is further pivoted upwardly to draw out the upper thread from an upper thread supply device. The pivotal stroke of the thread take-up lever is determined to a minimum distance required for performing this operation.
Here, it will be noted that each stitch sewn by a conventional sewing machine has a relatively short length. Although it has been proposed to adjust the length of the stitch to some extent, the adjusting range is relatively small. Therefore, when the pivotal stroke of the thread take-up lever is determined to correspond to an average stitch length, variations of the stitch length can be compensated to some extent by adjusting a tension adjusting device which is operable to adjust the tension of the upper thread, so that a proper thread-tightening operation can be performed.
On the other hand, each stitch sewn by a conventional embroidery machine has a relatively long length such as 0.1 mm to 12.7 mm, and in many cases, each stitch is controlled to have a different length from other stitches. Therefore, even if the pivotal stroke of the thread take-up lever has been determined to correspond to an average stitch length, the variations of tension of the upper thread may exceed the limit of adjustment of the tension adjusting device when the stitch length is too small or too large. This may cause excessive tightening of the thread or insufficient tightening, resulting in that the appearance of the stitches is degraded.